Avalanche Forecast for Sunday, February 23, 2020
This information was published 02/23/2020 at 7:07 AM.
The Bottom Line
Warming wind slabs are the primary concern today. Light wind, sunshine and increasing temperatures will warm wind slabs formed last week. Watch for moist snow at the surface as a sign that the likelihood of a human triggered avalanche is increasing. Avalanche danger is LOW overall, and will increase to MODERATE on sunny aspects receiving the most direct sun.
Continue to use safe travel habits including traveling one at a time, and limit your time in avalanche paths. Crowds will be a factor today, so be aware that people above may create a problem that becomes your problem.
Yesterday was clear with no precipitation or observed wind transported snow. The summit high temperature reached 14F.
Today we start the day at 18F and clear skies. Summit temperatures are expected to rise to the upper 20’s with W wind 20-35 mph. Sunny, lower elevation areas will rise above freezing. Clear skies will become partly sunny later in the afternoon. Tonight, temperatures will fall to upper teens.
Tomorrow, mostly cloudy skies with temperatures in the mid 20s F and west wind 40-55mph. Looking ahead, Tuesday night and Wednesday brings the next chance for new snow.
Primary Avalanche Problem – Wind Slab
Wind slabs formed last week have so far been proven to be firm and unreactive. Triggering these slabs is unlikely, though may become possible due to rapid warming as unseasonably warm temperatures and bright sun affects the snowpack. Warming today is a clear red flag that instability is increasing.
What is a Windslab Avalanche?
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Secondary Avalanche Problem – Wet Loose
It will be possible to trigger a Wet Loose avalanche at lower elevations today, though lack of a developed snowpack limits concerns. Watch for rollerballs, or moist snow. An avalanche at lower elevations may be small, but have significant consequences if it pulls you over a cliff in Crawford Notch. Sunny aspects at mid-elevations may also warm to the point that wet sluffing occurs though will likely be more limited due to less intense warming.
What is a Wet Loose Avalanche?
Wet Loose avalanches are the release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry Avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
The topic of our forecaster’s discussion this morning revolved around the amount of warming the snowpack will receive from ambient air temperature and solar gain. Both the MWObs and the NWS agree that the range will see unseasonably warm temperatures, light winds and clear skies for most of the day. If the summit reaches the upper 20’s, the ravines will likely reach temperatures above freezing, and south facing slopes may warm to the point of moist snow on the surface. This rapid warming, combined with observed faceting and the fact that south facing slopes did not avalanche during the last natural avalanche cycle drives our concerns for these aspects.
The Sherburne and Gulf of Slides ski trails are snow covered to Pinkham Notch and are skiing well.
Details on daily snowfall totals, precipitation type, total depth of snow and other information can be found on our page devoted to snow study plot data. Click here to check it out.
Recent snowpack and avalanche observations can be found here and on Instagram. Your observations help improve our forecast product. Please take a moment and submit a photo or two and a brief description of snow and avalanche information that you gather in the field.
Safe travel in avalanche terrain requires training and experience. This forecast is just one tool to help you make your own decisions in avalanche terrain. You control your own risk by choosing where, when, and how you travel.
Avalanche danger may change when actual weather differs from the higher summits forecast.
For more information contact the US Forest Service Snow Rangers, AMC visitor services staff at the Pinkham Notch Visitor Center, or the caretakers at Hermit Lake Shelters or seasonally at the Harvard Cabin (generally December 1 through March 31). The Mount Washington Ski Patrol is also available on spring weekends.
Posted 02/23/2020 at 7:07 AM.
USDA Forest Service
White Mountain National Forest